The biochemical properties (antitumor, carcinogenic and cardiotoxicity) of the anthracyclines may be related to DNA binding and or the production of oxygen derived toxic free radical metabolites. We have examined the formation of superoxide from these drugs and have evaluated the role of oxygen derived toxic species in their biological properties. Our findings indicate that formation of superoxide is not related to the antitumor activity. However, lipid peroxidation caused by superoxide or hydroxyl free radical is related to the cardiotoxicity of the anthracyclines.